1. Field of the Invention
The present invention relates to a CDMA (Code Division Multiple Access) Communication System, and more particularly to a frequency control method for controlling the frequency of channels established between the base station ad a mobile station.
2. Description of the Related Art
In recent years, the CDMA communication method, which is effective in controlling interference and disturbance, is receiving increasing attention as a communication method used in mobile communication systems. In such a CDMA communication method, a user signal that is to be transmitted is spread by a spreading code on the transmission side to be transmitted, and the original user signal is obtained on the receiving side by using a spreading code identical to the spreading code for despreading.
In the CDMA communication system, a plurality of transmission sides effect spreading by using different spreading codes each having orthogonality, and at the receiving sides, each communication can be identified by selecting the spreading code which is used at despreading, thereby enabling a plurality of communications sharing the same frequency band.
However, since it is difficult to maintain absolute orthogonality between all of the spreading codes that are used, each of the spreading codes are actually not perfectly orthogonal and have correlation components with other codes. These correlation components become interference components in that communication and bring about deterioration in communication quality. This produces an interference component which increases with an increase in the number of communications.
In general, a certain signal-to-noise ratio is needed in a radio communication system to secure communication quality. For example, in a spread spectrum communication system, a certain signal-to-(noise+interference) ratio is needed to secure communication quality. This value is usually referred to as Eb/I0, where Eb is a desired reception wave power and I0 is an interference wave power. The Eb/I0 needed to secure the certain communication quality is also referred to as a required Eb/I0.
As described hereinabove, however, for one particular receiver, transmission signals sent to another receiver are noise components and generate interference to the desired signals transmitted to the receiver. As a result, Eb/I0 can be secured most efficiently when the powers of a plurality of transmission signals received at a receiver are all the same. For this reason, in a CDMA communication system, the base station controls the transmission power of each mobile station such that the Eb/I0 obtained from the communications of each of the mobile stations reaches a reference Eb/I0. The reference Eb/I0 is a value that is set based on a required Eb/I0, and is typically substantially equal to the reference Eb/I0.
Specifically, the base station instructs each mobile station to increase or decrease the present transmission power based on transmission power control signals for instructing the increase or decrease of transmission power to mobile stations contained in the downward channel transmitted from the base station to mobile stations.
However, when a failure occurs at a mobile station, the control of transmission power may be impossible, and in some cases transmission power continues to rise regardless of the transmission power control by the base station.
When this happens, the reception Eb/I0 of mobile stations other than the mobile station in which the failure occurred is degraded by the influence of the communication of the mobile station in which failure occurred, so that the reference Eb/I0 can no longer be secured. Therefore, the base station instructs each mobile station in which failure has not occurred to increase transmission power of the mobile station, and each mobile station performs communications at a transmission power greater than needed at each mobile station, thus shortening the battery life. In a worst case, all mobile stations within a cell communicate at the greatest transmission power that can be transmitted. As a result, the required Eb/I0 cannot be obtained at all mobile stations within the cell, making all mobile stations within the cell lose the ability to communicate.
In other words, the CDMA communication system of the prior art has suffered from problem that, should control of transmission power be lost at any particular mobile station and transmission power rise to an unnecessarily high level, transmission quality at other mobile stations within the cell can no longer be maintained.